Selection of SPDs and their installation rules are generally not complex. For most cases, the path to follow is clear and straightforward. In a few circumstances, there are complexities either for selection of the appropriate SPDs (such as coping with superimposed high frequency impulses, switching surges, high temporary overvoltages, etc.) or more generally during the installation process (lack of space in a panel board, need to install SPDs in another place than originally scheduled, difficulties keeping lead length short, etc.). There is then a need to accommodate these complexities by selecting other SPDs (such as with a better voltage protection level) or adding more SPDs to further reduce the overvoltages in front of equipment to be protected. Except in these cases, the rules are simple (it does not mean that application of these rules is always simple). However, these rules are based on the assumption that protection is efficient. Even if true in a large majority of cases, application of standard rules may not be sufficient in a few cases. Sometimes (see Section 8.8), it is necessary to demonstrate protection efficiency. Then tests or calculations are needed and that makes the selection process more complex.